Use of glass ceramic plates as cooktops in electric cooking apparatus is becoming increasingly common. Among the advantages of this smooth cooking surface is its pleasing appearance and easy cleanability. However, due to the high thermal impedance of the glass ceramic plate, such cooktops are less efficient thermally than conventional cooking surfaces using sheathed heating elements.
Due to unique electrical and thermal characteristics possessed by materials such as molybdenum disilicide (MoSi.sub.2) and tungsten, resistive heating elements made from these materials are attractive for use with glass ceramic cooktops. The high positive temperatute coefficient of resistivity, low thermal mass, and low specific heat characteristic of MoSi.sub.2 and tungsten and the high operating temperature achievable using heating elements made from these materials provide the potential for improved thermal efficiency for cooking apparatus which incorporate a glass ceramic cooktop. However, these same dynamic electrical and thermal characteristics create power control problems which have thus far rendered the use of heating elements made from these materials impractical in electric cooking apparatus.
Conventionally, power control in electric cooking apparatus is achieved using temperature sensitive switches, such as bimetalic infinite heat switches. In operation, the operator adjusts the switch to provide the desired cooking temperature. The switch remains closed until the heating element reaches a predetermined temperature. The switch then opens and remains open until the element temperature drops to a predetermined temperature. The switch continues to cycle ON and OFF in this manner indefinitely. Since conventional sheathed heating elements heat up and cool down relatively slowly, these switching cycles are relatively long, ranging from a few seconds to thirty seconds. In addition, the resistance of a conventional sheathed heating element changes only slightly in going from room temperature to operating temperature. Since the resistance of conventional heating elements is relatively independent of temperature in the temperature range of interest, transient current surges when the switches close are minimal. Thus, conventional power control techniques work satisfactorily.
However. the dynamic characteristics of resistive heating elements made from MoSi.sub.2 or tungsten prevent these heating elements from being controlled, using conventional control techniques. Firstly, a MoSi.sub.2 heating element, as described generally in U.S. Pat. No. 3,912,905, designed for use in a cooking appliance, typically varies in resistance from 1 to 11/2 ohms at room temperature to 12 ohms at an operating temperature of approximately 1000.degree. C. Thus, assuming energization from a standard 120 volt AC household supply, as the temperature of the heating element changes from room temperature to operating temperature, the load current changes from an initial peak of roughly 110 amps to a steady state current on the order of 8.5 amps RMS. This initial current of 110 amps is obviously greater than can be tolerated in a household appliance except for extremely brief periods. Secondly, the heating element cools extremely rapidly; the first time constant for thermal response of this heating element being in the 600-1000 millisecond range. Since the element cools rapidly with a concurrent drop in resistance, excessive current surges may occur even during steady state operation because the resistance of the element may drop between applications of power to a level which draws excessive current during each subsequent application of power. Therefore, a very rapid switching capability which enables the use of brief ON times to limit the duration of excessive current during the heat-up of the element and brief OFF times to prevent unacceptable drops in resistance during steady state operation by limiting cooling of the element between ON times is required to avoid frequent excessive current surges.
Clearly, the relatively slow mechanical switching of the conventionally employed infinite heat switches cannot provide the rapid switching required to prevent excessive current flow during each application of power. Similarly, conventional electronic controls for use with conventional heating elements in cooling appliances have been designed to employ relatively long ON and OFF periods.
The above-described current surge problems for a hotplate type apparatus employing a single MoSi.sub.2 type resistive heating element are addressed in concurrently filed, commonly-assigned applications Ser. No. 8,376, filed in the name of Thomas R. Payne, entitled "Power Control for Appliances Using High Inrush Current Element," and Ser. No. 8,376, filed in the names of Thomas R. Payne and Alfred L. Baker, entitled "Repetition Rate Power Control for Appliances Using High Current Inrush Elements," which disclosures are hereby incorporated by reference. The control circuits disclosed in the above-noted applications may be used in an electric range employing multiple heating elements, four heating elements in a range cooktop being a common arrangement. In such an arrangement, a control circuit is provided for each heating element which operates independently of the others. However, the potential total current overload problems which result due to the use of multiple elements, both during transient start-up operation, steady state operation and those instances where one or more elements is operating in steady state operation and one or more is undergoing transient start-up operation, are not addressed in the above-noted applications. In the event all four elements are undergoing start-up simultaneously, total peak currents on the order of 440 amps could be drawn by the system. In addition, the transient surge current drawn by even a single heating element operating in the Soft Start mode may overload the system when added to the current drawn by other elements operating in the Steady State. Finally, when several elements are operating in the Steady State mode, the total current surges drawn by the system may interfere with power service to other electrical devices in the home, causing disturbing effects such as flickering in the intensity of room lighting.